Current mirrors commonly are used in electronic circuits. The input to a current mirror is a current and the output ideally is a current of identical magnitude or of a magnitude that is a predefined ratio of the input current. As used herein, an output current that is proportional to an input current includes the condition in which the output current is designed approximately to be equal to the input current and the condition in which the output current is designed to be a predefined ratio of the input current.
A basic current mirror designed in a complimentary metal-oxide semiconductor (CMOS) process is shown in FIG. 1. Current mirror 10 comprises a supply voltage VCC coupled to diode-connected input transistor 12 and output transistor 14, wherein the gates of transistors 12 and 14 are connected and the characteristics (e.g., threshold voltage) of the transistors ideally are identical. Responsive to input current IIN flowing through input transistor 12, current mirror 10 generates output current IOUT that ideally is equal to the magnitude of input current IIN or a predefined ratio of the input current.
A common challenge in the design of a current mirror is to obtain an output current that is as close as possible to the input current or to the predefined ratio of the input current. Mismatch of threshold voltages (VT) and transconductances (β) of transistors 12 and 14 may result in output current IOUT that is not identical to the input current or to the predefined ratio of the input current. At low input current levels, where the |VGS|−VT overdrive voltage is small, the threshold voltage mismatch typically dominates and leads to inaccuracies. At higher input currents, when the |VGS|−VT overdrive is greater, the threshold mismatch is less significant and the transconductance mismatch (Δβ/β) usually dominates. The transconductance of, e.g., an enhancement-type MOSFET is β=μCOX*W/L, where μ is the mobility of the electrons in the induced channel, COX is the capacitance per unit area of the gate-to channel capacitor, L is the length of the channel, and W is the width of the channel.
To reduce the threshold voltage mismatch and thereby improve the accuracy of the current mirror, degeneration resistors R1 and R2 may be coupled between supply voltage VCC and the sources of transistors 12 and 14 (respectively), as illustrated in FIG. 2. If the mismatch between resistors is less than the mismatch between the transistors and a voltage comparable to the nominal threshold voltage of transistors 12,14 appears across them, the overall accuracy of the mirror is improved. Alternatively, in a CMOS process, the matching between the transistors can be improved by increasing their lengths (L). This increases the |VGS|−VT overdrive and reduces the effects of threshold voltage mismatch. As used herein, the term “nominal threshold voltage” refers to the ideal threshold voltage for which a given transistor is designed. While two transistors may be designed to have the same nominal threshold voltage, manufacturing limitations may cause mismatch of the threshold voltages, thereby potentially causing the current mirror to generate an output current that inaccurately mirrors the input current.
While adding resistive degeneration or increasing the lengths of CMOS devices improves current mirror accuracy, both methods reduce the current mirror's dynamic range. If a resistor value or transistor length is chosen to improve the accuracy for low input current magnitudes, a larger input current magnitude results in a larger voltage drop across the resistor or transistor. When this voltage drop becomes comparable to total supply voltage VCC, the current mirror ceases to be useful. Ultimately, this limits the range of currents that can be mirrored accurately.
In view of the foregoing, it would be desirable to be able to provide methods and circuits for mirroring input current that improves the accuracy of the output current over a wide range of input current.
It further would be desirable to be able to provide methods and circuits for mirroring a wide range of input current, wherein the current mirror circuit automatically (1) detects an increasing magnitude of input current and (2) adapts the current mirror to accommodate the increasing magnitude.